The life cycle of internal waves in the ocean: effects of currents and modulational instabilities

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Internal waves are examples of nonlinear-dispersive waves, the study of which has been a fascinating and important area of research in fluid dynamics for over 200 years. They exist in the interior of density-stratified fluids when gravitational restoring forces act on vertically displaced fluid. The source of close to half of the internal wave energy in the oceans is tidal currents pushing fluid up and down sloping bathymetry such as the continental slopes and ocean ridges and in recent decades a lot of effort has been expended in understanding the spatial and temporal distribution of this wave source and on the fate of the waves as they propagate away. Internal waves play a fundamental role in transferring energy from large to small scales and in the vertical mixing of heat and nutrients impacting ocean circulation and productivity. They can transport fluid and particles over large distances. Their currents can re-suspend sediments and impact offshore structures. Fish have been observed to migrate vertically in response to the passing of large internal waves to optimize their feeding strategy. Internal waves can propagate thousands of kilometers from their generation site so wave generation in one location can have an impact at great distances. Internal waves appear in many forms including vertically and horizontally propagating internal wave beams and horizontally propagating internal wave modes. Beams are generated at near-critical bottom slopes and have been hypothesized to play a role in shaping the continental shelf. Horizontally propagating waves include internal tides with wavelengths of O(100) km which can evolve into internal solitary waves. My research objectives are to (i) determine how background currents affect internal wave generation by tide-topography interactions and how internal waves interact with ocean currents and eddies; (ii) investigate cross-shelf transport by internal waves; and (iii) to investigate the evolution of horizontally propagating internal wave packets to determine the conditions under which rogue (unusually large) internal waves may form and how the packets may break down. The research will use a mixture of two- and three-dimensional numerical modelling and theory. It will include idealized process studies and numerical simulations based on observations. Theoretical models will be used to understand the complex wave field simulated by these models and, because of their relative simplicity and speed, are often required to more fully explore a parameter space. Sites studied include the Angolan and Peruvian Shelves which are locations of important fisheries, and the Amazon Shelf. This research will contribute to the management of ocean resources and the improvement of our ability to predict the current and future state of the ocean. Members of my research team will gain valuable expertise in high-performance computing, including data analysis and visualization, and skills in working on complex problems.